MODIFICATION OF THE CONTINENTAL CRUST BY SUBDUCTION ZONE MAGMATISM AND VICE-VERSA: ACROSS-STRIKE GEOCHEMICAL VARIATIONS OF SILICIC LAVAS FROM THE MOGOLLON-DATIL VOLCANIC FIELD, SW NEW MEXICO

Continental arc volcanoes represent a dramatic surface expression of one of the most significant and fundamental phenomena in global tectonics: the subduction of an oceanic plate beneath a more buoyant continental plate. The subduction of an oceanic plate results in recycling of crustal material into the convecting mantle, partial melting, and primary basalt production. Moreover, during passage through thick continental crust, subduction zone magmas may substantially differentiate and melt crustal rocks giving rise to the great diversity of igneous lithologies characteristic of earth. These are important processes that must be understood in detail in order to interpret the long-term evolution of the earth and continental crust.

In this project we compare variations in the isotopic and trace element composition of andestites and basaltic lavas that are between 38 Ma and 27 Ma located throughout the Mogollon- Datil volcanic field (MDVF) of central New Mexico. At a given SiO₂ content, volcanic rocks erupted with increasing distance from the arc front display systematically higher K₂O, Rb, Y, REE and HFSE contents; Rb/Sr ratios; and Sr isotopic ratios. In contrast, the volcanic rocks should display systematically lower Al₂O₃, Na₂O, Sr, and Ba contents; Ba/La, Ba/Zr, K/Rb, and Sr/Y ratios; Nd isotopic ratios; and more negative Eu anomalies away from the arc front. We suggest geochemical trends in silicic magmas compositions in the southwest MDVF reflect melting of relatively young, mafic composition source rocks and that the mid- to deep-crust becomes increasingly older with a more felsic bulk composition in which residual mineralogies are progressively more feldspar-rich toward the east.